Low school rank makes me very concerned for your Adv Eng mark which always counts.
For your high rank subject, it's hard to tell with your school rank how strong you really are in a subject. You be an absolute gun in it and kill the HSC, or only be the best at your school whilst at a 100 rank school you'd be 30th.
So it's all down to how you go in externals. You have the potential to get a really high ATAR if you kill externals.
But as an estimate you'd be at least looking at low 90s. However you are in the position to get higher than that esp. with external HSC results.